(a) Field of the Invention
The present invention relates to a steel plate with a layered structure. More specifically, the present invention relates to a method for manufacturing the steel plate with a layered structure ad that is capable of obtaining strength and ductility together.
(b) Description of the Related Art
A lot of steel is used in a car. Therefore, it is necessary to reduce weight of the car by reducing usage of steel in order to improve fuel consumption efficiency of the car or reduce discharging gas. However, the strength of the steel must be obtained with ductility thereof in order to secure safety of the car against an outer impact. Therefore, it is necessary to develop steel that is capable of obtaining strength and ductility together.
For this, martensitic steels such as DP (dual phase) steel and TRIP (transformation induced plasticity) steel have been developed as steel for cars. Ductility of DP steel or TRIP steel is increased by precisely controlling microstructures thereof. The martensitic steel cannot be used as steel for cars since ductility thereof is low even if it shows high strength of not less than 1000 MPa. Therefore, ductility of the martensitic steel should be increased when used as steel for cars, but although methods for designing alloys, removing impurities, and controlling microstructures are performed to control properties of the steel, these attempts have reached a limit.